The marmoset, a small South American primate, is a unique animal in which to study the early embryology of multiple births, and the differentiation of germ cells from early fetal life through adulthood. The marmoset normally produces biovular twins which show complete chorionic fusion prior to somite formation. The embryological processes which lead to blastocyst fusion and early blastocyst form, i.e., prior to fusion, are unknown. It is the aim of this research to describe the pre-fusion blastocyst, determine how and when blastocyst fusion occurs, and to define the normal development of the unusual bidiscoidal placenta. Inter-twin placental vascular anastomoses develop and allow the exchange of cells which migrate or are motile during embryonic development. This gives rise to individuals which are chimeric for hematopoietic, lymphoid, and, most especially, germinal tissue. Thus, we have available a primate model in which to examine sex-determining factors and in which to evaluate the roles of somatic and germinal elements in gamete differentiation. This is being carried out by investigating germ cell chimerism in spermatogonial and oogonial mitosis and spermatocyte and oocyte meiosis. Data indicates that approximately one-half of all males possess germ cells with an XX sex chromosome constitution; these germ cells appear functional at least through the diakinesis phase of meiotic prophase. Preliminary evidence indicates germ cell chimerism also occurs in female marmosets; oocytes which possess a bivalent which resembles the XY bivalent in end-to-end association have been demonstrated.